Systems which handle material, such as manufacturing facilities, often employ automated material handling systems to move various materials from one location to another location within the system. The materials may include raw materials, finished product materials, or any materials in between. It is common to employ containers or carriers to move the materials from one location to another during the manufacturing process. Typically, a transportation system is employed, such as a conveyor belt system, for moving the carriers from one processing location to another within the system. Typically, empty containers are needed at the various process locations for holding processed materials. Likewise, empty carriers are generated as material is removed from a carrier in order to be processed. Empty containers may also be added or removed from the system.
An example of such a manufacturing facility is a facility for fabricating integrated circuits on semiconductor wafers. The facility may include a plurality of wafer fabrication process tools which perform various fabrication process steps on lots of semiconductor wafer lots. Each process tool typically has an accompanying stock area for storing carriers of wafers waiting to be processed by the tool or having already been processed by the tool. Wafer carriers may also be referred to as cassettes or “boats”. The process tools need empty wafer carriers, for example, holding processed wafers after a process tool has performed a wafer fabrication operation. Conversely, as process tools remove wafers from a carrier for processing, empty carriers are generated.
Typically in an integrated circuit manufacturing facility, the requests for moves of empty carriers is very non-linear. The non-linearity is introduced, for example, by the start of the processing of many new wafer lots which suddenly creates the need for a large number of empty carriers. In addition, a situation may occur in which a process tool fails and causes a “logjam” or “bubble” in the system, thereby consuming a large number of empty carriers, and preventing empty carriers from being generated. In such a situation, empty carriers become a precious resource.
Typically, if an empty carrier is not available when needed by a process tool, for example, an empty carrier must be moved from another location in the system to fulfill the need for the empty carrier. The move of the empty carrier from the other location to the location where the empty carrier is needed imposes a time delay in the process of fabricating a wafer lot. The time delay may introduce inefficient use of the process tool needing the empty carrier. The cumulative cost of many such inefficiencies can be substantial.
One potential solution to the problem is to provide a relatively large number of empty carriers in order to minimize the delay times. This solution may be prohibitive both in terms of storage costs and carrier costs. Therefore, an improved method for managing empty carriers in an automated material handling system is desired which reduces cost by reducing the number of empty carriers required, reducing the number of carrier moves in the system, reducing the number and/or size of stock areas, and improving the production efficiency of the system.